"Nothing's changed with regard to Force of the Future," Pentagon Press Secretary Peter Cook said of the plan that has been trumpeted as a legacy initiative for Defense Secretary Ashton Carter.

"The secretary remains squarely behind the initiatives he's already unveiled and has more to come," Cook said. Carter will be making the case for Force of the Future in upcoming congressional testimony, Cook said.

"And he feels confident that these are changes that are critical to this department going forward" and adapting to retain personnel and recruit more into the ranks in a changing economy, Cook said. "It's our goal to continue working with Congress."

Force of the Future suffered a major setback Monday with the resignation of Brad Carson, the Pentagon's top personnel chief as acting Under Secretary of Defense for Personnel and Readiness.

Carson, considered the "architect" of Force of the Future, was expected to leave the building on April 8. Morgan Plummer, a former Army officer and Carson's top adviser, was expected to leave along with Carson, Pentagon officials said.

In a statement after Carson announced his resignation, Carter said, "Brad Carson has developed some of the most important and groundbreaking work in years to modernize our personnel policies."

"At my direction, he charted a path forward for the Department and our people that will leave a lasting legacy, and will improve the mission effectiveness, readiness and the quality of life for our civilian workforce, uniformed service members and families," Carter said.

Carson was nominated last year by President Obama to succeed Jessica Wright in the Under Secretary's personnel post but immediately ran afoul of the Senate Armed Services Committee over alleged violations of the obscure Federal Vacancies Act, as well as the reforms he was proposing.

The resignations of Carson and Plummer followed a particularly confrontational hearing before the Senate Armed Services Committee (SASC) at which the Force of the Future proposals were called "outrageous" and Carson's qualifications were questioned.

"This initiative has been an outrageous waste of official time and resources during a period of severe fiscal constraints," Sen. John McCain, an Arizona Republican and the SASC chairman, told Carson. "It illustrates the worst aspects of a bloated and inefficient defense organization."

"I find it deeply disturbing that you are proposing to add expensive fringe benefits allegedly aimed at retention during a time when we are asking 3,000 excellent Army captains to leave the service who would have otherwise chosen to remain on active duty," McCain said.

McCain and other members of the panel also expressed concerns that Carson had violated the Federal Vacancies Act by performing the duties of the undersecretary for personnel and readiness while still in the "acting" status.

The Federal Vacancies Act had also tripped up the nomination of Eric Fanning, the acting Secretary of the Army, to become the first openly gay head of a service branch. The Pentagon got around that by changing Fanning's title and his nomination was approved by the SASC earlier this month to become secretary of the Army.

However, Fanning's nomination was still on "hold" because of Sen. Pat Roberts, a Kansas Republican. Roberts was seeking assurances from the White House and the Defense Department that Guantanamo Bay, Cuba, prisoners will not be transferred in the future to Fort Leavenworth, Kansas, before he will lift the hold on Fanning's nomination.

Carson's reform proposals under Force of the Future, which he billed as the "biggest personnel overhaul in 45 years," were aimed at fundamental changes in the way the Defense Department recruits, pays, promotes and manages the military.

In an article for Defense One last June, Carson bluntly described what he was attempting in Force of the Future as an effort to discard the hidebound ways of doing things that had become entrenched in the Pentagon bureaucracy.

"In this modern, data-informed world, neither the business community nor the Defense Department has time or decision space for the bureaucratic needs of hierarchical, linear organizations of a former era," Carson said.

"Unfortunately, the department has been just that type of institution when it comes to personnel management -- until now," Carson said.

The major personnel changes sought by the Pentagon require the approval of Congress. Other changes can be authorized by Carter without the input of Congress.

With only 10 months left in the Obama administration, it remained unclear whether the major changes could be put in place this year, or whether Carter would turn to seeking to lay the groundwork for reform for the next president.

Critics of the plan question the viability of adding personnel programs that could potentially cost billions at a time when the Defense Department was cutting the Army to 450,000 troops and also was continuing to face the possibility that the arbitrary budget caps known as sequestration could be re-imposed.

One of the main proposals in the Force of the Future plan that would require congressional approval would revamp the military pay system by creating new basic pay tables for high-demand career fields such as cybersecurity to allow commanders to dole out merit-based cash bonuses.

Another major proposal, also requiring congressional approval, would change the "up-or-out" system of promotions under federal law that limits the number of times an officer can be passed over for promotion before being forced to leave the military. Under Force of the Future, promotions would be based more on experience and performance rather than time in grade.

Carter has unveiled the proposed Force of the Future changes in three "tranches," which were detailed by Military Times. The first tranche disclosed in November included:

-- Creating a new high-tech personnel management system for matching individual troops with job assignments in an online network Carter compared to Facebook.

-- Creating a new civilian "chief recruiting officer" to oversee efforts to attract top talent.

-- Expanding corporate fellowship programs to allow service members to work in the private sector.

-- Expanding options for military service members seeking specific duty stations to provide more stability for their families.

Carter has also been considering a "tranche three" and possibly a "tranche four" of personnel changes. One of the additional reforms Carter was expected to propose involved changing regulations to make it easier for mid-career civilian professionals to seek "lateral entry" to join the military without having to start at the bottom of the traditional rank and pay structure.

In his statement announcing his resignation, Carson said, "I'm very grateful to the men and women of the Office of the Under Secretary of Defense for Personnel and Readiness. Their commitment to those that serve our country, in and out of uniform, is unparalleled, and I am deeply grateful for the opportunity to have served with such an outstanding team."